The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, a torque converter of an automatic transmission includes an impeller, a turbine, and a stator as basic components and further includes a lockup clutch capable of directly and mechanically transmitting a torque in a high-speed section.
For example, the lockup clutch is disposed in a space formed between a front cover which is an input-side rotating member of the torque converter, and a turbine which is an output-side rotating member thereof and includes an internal pressure chamber together with a damper hydraulic chamber.
The damper hydraulic chamber is operated as a hydraulic chamber for a lockup clutch engagement so as to implement a direct torque transmission from the front cover to the turbine. The internal pressure chamber controls the line pressure to be decreased so as to deliver a control pressure adjusted by a solenoid valve of a hydraulic line with an appropriate line pressure.
Therefore, the lockup clutch improves the line pressure increased by an internal pressure of the torque converter together with the torque transmission during an ON or OFF operation of a damper. Consequently, when the damper is operated, the torque converter may use an internal pressure applied to a piston, which is about 3 bars lower than that of the lockup clutch not applied to an internal pressure chamber.
However, we have discovered that there is a difficulty in that a line pressure down control should be performed without degradation in operating performance of the torque converter.
For example, since the internal pressure chamber of the lockup clutch is focused to decrease the line pressure, we have discovered that the following problems are caused. First, a loss of an internal flow rate of the torque converter may be caused while the line pressure is decreased, and thus there occurs a problem in that a lubrication flow rate is decreased relative to a transmission to the loss of the internal flow rate. Second, air may flow into the internal pressure chamber of the lockup clutch when the internal pressure chamber is left for a long period of time, and thus there occurs a problem of malfunction of a damper piston due to a centrifugal pressure.
Therefore, the line pressure down control of the torque converter should be capable of decreasing a line pressure, while reducing a loss of an internal flow rate, and inhibiting an inflow of air.